1. Field of the Invention
The present invention relates to an activating catalytic solution for electroless plating and a method of electroless plating utilizing the same.
2. Description of the Related Art
Electroless plating is sometimes used for forming a conductive film having a predetermined pattern for a coil for high frequencies, dielectric filter, capacitor, hybrid IC or the like, on an base such as an alumina substrate, dielectric ceramic substrate, polyimide substrate, glass epoxy substrate or ferrite substrate. When electroless plating is performed, the surface of the base must be activated, and an activating catalytic solution is used for such activation.
The activating catalytic solution is applied to the base to form a photoreactive film on the base. Attracting attention are activating catalytic solutions for forming such a photoreactive film in which activation selectively takes place only in regions irradiated with ultraviolet rays or laser beams and, therefore, electroless plating can be selectively performed only in those particular regions. With such an activating catalytic solution, a conductive film having a fine pattern can be efficiently formed on the base substrate using photolithography.
As an activating catalytic solution which provides the above-described feature, for example, a product obtained by dissolving palladium acetylacetonate into an organic solvent such as chloroform has conventionally been used. Such an activating catalytic solution is applied to an appropriate base to form a photoreactive film thereon which is in turn irradiated with laser beams or ultraviolet rays through a photomask. As a result, palladium metal is deposited on the base only in regions which are exposed. Thereafter, the photomask is removed. Photoreactive films in regions which have not been exposed are flushed away using an organic solvent such as chloroform. Thus, films made of palladium metal which have been left will be developed in a pattern corresponding to the pattern of the photomask. By immersing this base in an electroless plating bath, an electroless plating film is formed on the base using palladium metal as an activating catalyst. This process is hereinafter referred to as the "first conventional technique".
Meanwhile, a method of selective electroless plating utilizing an activating catalytic solution obtained by dissolving ferric oxalate and palladium chloride into a solution of potassium hydrate is described in "SELECTIVE PLATING OF COPPER FOR CIRCUITIZATION OF TEFLON AND EPOXY-BASED SUBSTRATES" by Thomas H. Baum et al. in "The Electrochemical Society Proceedings", Vol. 94-31 (hereinafter referred to as the "second conventional technique").
In the above-described first conventional technique, an organic solvent such as chloroform must be used in the developing step. Since such an organic solvent is harmful, it can not be readily used in large amounts. It is therefore relatively difficult to completely flush away photoreactive films which have not been exposed to the light. If any unexposed photoreactive film remains in place, there will be a problem in that the clearness of the pattern of the plating film produced by subsequent electroless plating will be lost. There is another problem in that the exposure involves high cost because the exposure of a photoreactive film requires a light source having high energy such as an excimer laser and takes time.
On the other hand, the developing step according to the second conventional technique can be carried out using water because hydrophilic palladium chloride is used. It is therefore easy to completely flush away photoreactive films which have not been exposed using a large amount of water. However, the photoreactive film obtained by this second conventional technique has a low sensitivity to exposure and, therefore, the exposure of the same also takes time. Further, a metal film obtained by means of electroless plating using such a photoreactive film as an activating catalyst has a problem in that it has a relatively low electrical conductivity which reduces high frequency characteristics. In addition, such an electroless-plated metal film has another problem in that the strength of its adherence to the base is relatively low. Although the strength of adherence can be improved by etching the surface of the base, such etching can be difficult depending on the quality of the base.